add a feature flag for course/account level item banks
add a feature flag for course/account level item banks

For the future work on course/account level sharing
of item banks in new quizzes we need to create this feature
flag.

flag = none
refs QUIZ-8354

Test Plan:
 - Specs pass
 - Nav to the site admin feature flags
 - Should be able to see the new flag

Change-Id: I8368be90fb0ee7b105798a14b53a06f7cd6bacde
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/263937
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Mark McDermott <20dd8ba615f2257ea7254f93d6f30ae11aa4f904@instructure.com>
Product-Review: James Logan <b269b2a65e0517563b89a77ece857ee0ef50f5f3@instructure.com>
Reviewed-by: Jared Crystal <06625ed3fabb97fffd807b5fd3c4e9314fe77676@instructure.com>
Reviewed-by: Jorge Arteaga <797aca0b58c1608d2f0b1cb4d451b10108621a94@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms